The present invention is directed to a method of and an apparatus for the electrodeposition of a metal coating on a metallic endless belt, such as a press belt for use in a double band press. The belt to be coated is wired as an cathode and is located in an electrolytic bath with an anode so that the electrolyte dissociates into ions with the ions containing the metal atoms forming the coating. The cathode and the anode are connected to the corresponding poles of a constant voltage sourse.
Such belts, used chiefly as press belts in double band presses, are utilized for applying surface pressure on sheetlike workpieces, such as decorative laminated materials, chip boards, fiber plates, electro-laminates and the like. The material being pressed is directed between two continuously circulating endless press belts and pressure is directed against the belts and, if necessary, heat is applied so that the material being pressed is hardened, note German Offenlegungsshrift No. 24 21 296. Normally, such press belts are produced from high-tensile steel.
To prevent the press belts from wearing out too quickly due to the pressure applied during pressing, a hard metallic and wear-resistant layer is electrodeposited onto the surfaces of the press band. If the surface of the material to be pressed is to be provided with a texture, embossing bands are used, similar to the press belts, which consist of a steel band and a soft metallic layer is deposited on the surface of the belt and the layer is provided with the desired texture. A hard layer is then deposited on the soft layer for its protection, note German Patentschrift No. 29 50 795.
To electrodeposit a metal coating on a metallic object, tub-shaped baths are utilized filled with liquid electrolyte which dissociates into ions containing the desired metal atoms to be deposited. An anode, formed of a material with good conductivity, is immersed or dipped into the electrolyte. The item to be coated is completely submerged in the bath and is connected as a cathode. If a constant voltage source is provided outside the bath and is connected to the cathode and the anode, current consisting of the electrolyte ions flows in the bath between the cathode and the anode and the metal ions are transformed at the cathode by the reception of electrons into metal atoms which deposit out on the cathode as a metallic coating.
The baths used in electroplating shops have certain maximum lengths and depths. For entirely accommodating an endless belt in such a bath or tank it has been known to fold the belts and then introduce them into the bath. With this method, however, only belts up to approximately 6 m in length, corresponding to an annular diameter of approximately 2 m, can be coated in the largest available baths. For various applications in double band presses, such as for continuous chip board production, it is necessary to construct long presses having a length of approximately 12 m or more so that the press band has a circumferential length of at least 26 m. Such elongated press belts, however, can no longer be coated conventionally by electrodeposition means.
Another disadvantage of electrodeposition in a conventional bath results from the folding of the press belts. Due to the folding of the belt the surface is located at various distances from the anode and the current density between the anode and the cathode in the bath varies considerably whereby a different thickness of the deposited metal layer on the belt surface results. The different thicknesses of the coating can lead to variations in thickness of the material being pressed and can require a costly secondary operation on the pressed material, such as grinding. Moreover, it has been noted that an increased hydrogen embrittlement takes place in press belts which are coated in conventional baths. Such hydrogen embrittlement leads to cracks and fractures in the deposited metallic coating and can result in the entire press belt being unusable.